Regulation of the reversible Ser-19 phosphorylation of 20 kDa myosin light chain (MLC) primarily governs the extent of vascular smooth muscle contraction. Although a rise in cytoplasmic Ca/2+ acts as the main triggering mechanism for phosphorylation of MLC by activating Ca/2+- calmodulin-dependent MLC kinase, physiological membrane and cytosolic receptor ligands, such as catecholamines and nitrovasodilators, exert their effect in large part by dynamically changing the Ca/2+ sensitivity of MLC phosphorylation and contractile force. The principle objective of this proposal is to determine the molecular mechanism by which receptor activation of vascular smooth muscle modulates the contractile Ca/2+ sensitivity and to use this information to explore the molecular and cellular mechanism of cardiovascular abnormality, such as hypertension. Particularly, we wish to further elucidate the mechanism of two novel signaling pathways regulating Ca/2+ sensitivity in vascular smooth muscle: protein kinase C (PKC)-induced inhibition and protein kinase G (PKG)- induced activation of MLC phosphatase. We shall determine (1) if PKC, through phosphorylation of a novel muscle-specific protein phosphatase inhibitor protein CPI-17, increase the contractile Ca/2+ sensitivity in a manner that indicates its major effect is to inhabit MLC phosphatase, (2) if, how much and at which site(s) in situ CPI-17 is phosphorylated in response to PKC activations and other Ca/2+-sensitizing agonists, (3) if subcellular localization of CPI-17 is regulated by PKC activators and other agonists and if this translocation is modified by various inhibitors, (4) if PKG indirectly activates MLC phosphatase to decrease the Ca/2+ sensitivity and alters the phosphorylation levels/site(s) of CPI-17, (5) if cGMP changes the CPI-17 translocation and PKG is co- localized with CPI-17 and/or PKC during the Ca/2+ desensitization.